Please Help Me (I am in trouble)

I have a strange situation.  I have recently set up a 55 gallon 
freshwater plant tank.  I have a soil/vermiculite/peat substrate covered 
with gravel.  The problem is that my PH is very high (8.0-8.5).  My tap 
water is extremely hard and has a 7.5 PH.  I cannot discover why my PH is 
so high.  I origionally thought that the gravel that I bought contained 
marble chips (I tested it and it had a PH above 9).  I then removed 
almost all of the old gravel and replaced it with new gravel (without 
marble chips).  I left the old soil in the tank and put the new gravel on 
top of it.  Now the PH constantly increases to 8.2 or more.  Even when I 
adjust the PH down, it raises again.  

Does anybody have any idea what is going on?
Since my PH is raising to 8 or more isn't the buffering effect of the 
water exhausted?  Why would it still increase?  
Please help me.  I am on the verge of tearing down the tank and removing 
my soil mixture and cleaning my gravel.

Thanks for any help that you might give.
Please respond if you have any idea, hypothesis, or strange dream about 
my problem.
Ken Wilson
Wils0153 at maroon_tc.umn.edu

PS My plants are having a great time.